1. Field of the Invention
The present invention relates to an ink-jet recording method for conducting recording by using a plurality of inks to discharge the inks on a recording medium, and an apparatus for use in such a method.
2. Related Background Art
In recent years, apparatuses for office automation such as computers, word processors and copying machines have widely spread. A great number of recording systems, have hence been developed for use in recording apparatus thereof. Ink-jet recording apparatuses have excellent features in that they readily achieve high resolution and are excellent in low noise generation even at high speed compared with other recording systems, and moreover are inexpensive. Needs for color recording are also increasing. Therefore, a great number of color ink-jet recording apparatuses have also been developed. In the inkjet recording apparatus, an ink is jetted from a nozzle to cause the ink to adhere on a recording paper sheet, thereby forming an image. The diameter of the nozzle is as small as about 50 to 100 .mu.m. Therefore, inks for ink-jet recording are added with a non-volatile and high-hygroscopic solvent so as to prevent the inks from evaporating and drying to clog the nozzle at its tip. However, such inks have no quick drying property on recording media after recording though they have an effect to prevent the clogging due to the deposition of dye(s) in a nozzle orifice because they become hard to dry owing to the addition of a non-volatile wetting agent.
Color ink-jet recording apparatuses involve a problem in that color mixing (boundary feathering) occurs at boundaries between an ink of a certain color and other inks of different colors due to diffusion of dyes contained in the individual inks, resulting in deterioration in image quality. This color mixing is caused by mixing of an ink, which has been discharged on a paper sheet and exists on and in the paper sheet in a state that it is not sufficiently dried and fixed, with another ink of a different color, which adjoins the former ink. This mixing is caused by diffusion of coloring matter (colorants such as dyes and pigments) in a liquid-liquid interface between different inks. Therefore, the color mixing particularly tends to occur at such boundaries between different colors when ink-jet recording inks having no quick drying property are used.
In order to impart quick drying property to inks used in ink-jet recording systems, it has heretofore been attempted to add a surface active agent into an ink, thereby lowering the surface tension of the ink to increase its penetrating power into paper so as to penetrate into paper, or to add a solvent relatively high in vapor pressure into an ink, thereby lowering the surface tension of the ink to increase its penetrating power into paper and drying the ink owing to the evaporation of this solvent. Alternatively, it has been proposed to use only paper sheets high in permeability even in the case where inks to be used have no quick drying property or to decrease (or thin out) the amount of an ink to be jetted by one scan and then repeat the scan several times at proper intervals (multipass printing). Furthermore, it has also been attempted to provide a fixing device to forcedly evaporate inks.
When the surface active agent or the solvent high in vapor pressure is added too much, however, the wettability to paper becomes higher. Therefore, dots spread in a direction parallel to the paper surface and the dot diameter becomes greater correspondingly, whereby the sharpness of their edges is lost. Furthermore, the dot density is also lowered because the penetrating power of the ink is increased, whereby the ink permeates to a greater extent in a direction perpendicular to the paper surface. Besides, the excessive addition of the solvent high in vapor pressure facilitates the evaporation of the ink and hence involves a drawback that clogging tends to occur.
If paper sheets to be used are limited to special paper sheets for exclusive use, there are drawbacks that consumers can not use those other than these paper sheets and such paper sheets are very expensive.
In the case of the multipass printing, there is a problem that the number of times of scan increases and printing time per sheet (throughput) becomes very long if an interval of time in scan is lengthened.
The use of the fixing device involves a drawback that the apparatus is made a larger size as a whole and there is a poor economy from the viewpoint of energy.
As described above, various drawbacks are involved in imparting the quick drying property to inks used in the ink-jet recording apparatus. Therefore, these must be well balanced upon designing an ink-jet recording apparatus. In order to achieve reduction in size, cheap cost and speedy recording of images, however, there is at least a problem that when an ink of a certain color adjoins another ink of a different color before they are sufficiently dried and fixed on a paper sheet, unintended mixing of coloring matter (boundary feathering) as described above occurs at their contact boundaries, resulting in an irregular and blurred image.